Test head for optically inspecting workpieces

ABSTRACT

Apparatus for simultaneously optically inspecting top and bottom surfaces of a workpiece comprise upper and lower test heads, each head comprising at least one laser for providing a laser beam that scans its associated workpiece surface and at least one detector for detecting reflected laser light. The directions of the laser beams are selected so as to reduce or prevent cross-talk interference between the upper and lower test heads.

CROSS-REFERENCE TO PROVISIONAL PATENT APPLICATION

This application claims priority based on U.S. provisional patentapplication no. 60/643,748, filed Jan. 13, 2005.

BACKGROUND OF THE INVENTION

This invention relates to methods and apparatus for optically inspectingworkpieces such as substrates used to make magnetic disks or magneticdisks during any point in the manufacturing process (including thefinished disk).

Magnetic disks are typically manufactured using the following method.

-   1. A disk-shaped substrate (typically an Al alloy) is lapped or    ground.-   2. A material such as a nickel phosphorus alloy is plated onto the    substrate.-   3. The plated substrate is polished and textured. (During texturing,    texture grooves are typically formed in the substrate by mechanical    abrasion to cause a subsequently deposited magnetic layer to exhibit    anisotropy. It is also known to laser texture substrates for    tribological reasons.)-   4. One or more underlayers, one or more magnetic layers and one or    more protective overcoats are deposited onto the plated substrate.    (The deposition process can comprise sputtering or other    techniques.) Other layers can also be deposited onto the substrate    during manufacturing.-   5. A lubricant is applied to the disk.

At various points during manufacturing (e.g. before or after texturing),it is desirable to inspect the substrate for bumps, pits, contaminantparticles, or other defects. During such inspection, one should be ableto detect very small defects. It is known in the art to use lasers toscan such substrates for this purpose. See, for example, U.S. Pat. Nos.6,566,674 and 6,548,821, issued to Treves et al. (The Treves patents areincorporated herein by reference.)

We have developed an improved optical test apparatus comprising upperand lower test heads for applying upper and lower laser beams to theupper and lower workpiece surfaces. The upper and lower test heads alsoinclude one or more detectors for receiving light reflected off of theupper and lower workpiece surfaces.

It is desirable to prevent or reduce “cross-talk” between the upper andlower test heads during testing.

SUMMARY

Apparatus constructed in accordance with our invention comprises firstand second optical heads for inspecting first and second surface of aworkpiece (typically simultaneously). The heads each include at leastone laser for applying a laser beam to an associated workpiece surfaceand at least one detector for receiving light reflected off of theassociated workpiece surface. (As used herein, “detectors” are opticaltransducers.) The heads can include other elements such as lenses,mirrors, polarizing plates, masks, etc.

The laser beams approach their associated workpiece surfaces at an anglethat is not perpendicular to the surfaces. The angles at which the laserbeams approach the workpiece surfaces have the characteristic that ifthe workpiece is not present, the light beam provided by the first headdoes not travel toward the detector in the second head, and the lightbeam provided by the second head does not travel toward said detector inthe first head. In one embodiment, the components of the directions ofthe laser beams associated with the first and second heads in the planeof the workpiece are typically close to antiparallel. In anotherembodiment, the components of the first and second directions are notclose to antiparallel, but are at an angle. This angle prevents orreduces optical cross-talk between the first and second heads.

Apparatus in accordance with the invention is used for inspecting asurface of a workpiece. As used herein, the term “inspect” includestesting a workpiece surface for the presence of defects; evaluating thesurface; collecting data concerning the surface of the workpiece; and/ordetermining whether the surface is suitable based on one or morecriteria. A “workpiece” is any object to be inspected.

In some embodiments, there is a plurality of output paths for receivinglight reflected by the workpiece. Typically, the head comprises betweenone and six output paths for receiving different types of reflectedlight.

In one embodiment a method and apparatus in accordance with theinvention are used to inspect substrates used for magnetic diskmanufacturing. However, the method and apparatus can also be used toinspect a magnetic disk at any portion during the manufacturing process,for example a) an aluminum substrate prior to being plated with NiP; b)the substrate after plating with NiP but before being polished andtextured; c) the substrate after polishing but before texturing; d) thesubstrate after texturing but before sputtering of the underlayer,magnetic layer and protective overcoat, e) the disk after sputtering butbefore application of a lubricant; or f) the finished disk. There areseveral points during which the disk is washed. Inspection can occurbefore or after washing. As used herein, the term “platter” encompassesa disk at any point during or after manufacturing (including disks madeusing non-aluminum substrates, disks made using deposition processesother than sputtering, disks used in longitudinal or vertical recording,disks used in conjunction with vertical recording, disks used inconjunction with longitudinal recording, textured disks and untextureddisks).

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B schematically illustrate optical inspection apparatusincluding an optical inspection head for inspecting a workpiece. FIG. 1Ais an exploded perspective drawing.

FIGS. 2A and 2B schematically illustrate the optical paths and opticalelements within the optical inspection head of FIGS. 1A and 1B. FIG. 2Ashows the optical paths and elements from a perspective view. FIG. 2Bshows the optical paths and elements in plan view.

FIGS. 3A to 3D are perspective views of a monolithic block of materialforming the optical inspection head of FIGS. 1 and 2.

FIG. 4 illustrates a platter having a surface with two scratchesthereon.

FIGS. 5A and 5B illustrate a block of material during a portion of amanufacturing process in accordance with the invention.

FIG. 5C illustrates a structure for mounting a mirror in one of theoptical paths.

FIG. 6A schematically shows the optical path between a platter and anoptical detector comprising two lenses for collecting and concentratinglight.

FIG. 6B schematically shows the optical path between a platter and anoptical detector comprising one lens that both collects and concentrateslight.

FIG. 7 illustrates two optical heads inspecting upper and lower plattersurfaces in accordance with an alternative embodiment of the invention.

FIG. 8 illustrates paths traced by upper and lower laser beams on aplatter.

FIGS. 9 and 10 illustrate the placement of lenses in a lower test headin accordance with the invention.

FIG. 11 illustrates the point on an innermost track where a laserstrikes a platter.

FIG. 12 illustrates the juxtaposition of the upper and lower heads andthe platter in plan view.

FIG. 13 schematically illustrates in plan view a quad detector fordetecting specularly reflected laser light within an embodiment of theoptical inspection head.

FIG. 14 schematically illustrates a circuit for processing outputsignals from the quad detector of FIG. 13.

FIG. 15A schematically illustrates a circuit for providing selected datacorresponding to the output signals of optical detectors to a processingdevice.

FIG. 15B schematically illustrates a circuit for providing selectedlocation information to the processing device.

FIG. 16A and 16B schematically shows circuitry details that can be usedin the circuits of FIGS. 15A and 15B.

FIG. 16C illustrates a logic circuit that can be used in conjunctionwith the circuit of FIGS. 16A and 16B

FIGS. 17A and 17B illustrate first and second embodiments of roboticcells used in conjunction with the test heads of the present invention.

DETAILED DESCRIPTION

I. Overview of Optical Inspection Apparatus 10

Apparatus in accordance with our invention comprises two optical testheads for inspecting first and second surfaces of a workpiece. Thisdetailed description begins with a description of an apparatuscomprising one test head. Thereafter, apparatus comprising two testheads and the manner in which they are arranged are described.

FIGS. 1A, 1B, 2A and 2B schematically illustrate an example of anoptical inspection apparatus 10 that includes a head 12 for opticallyinspecting a top surface 13 u of a workpiece 13 (typically a platter).(FIGS. 1A and 1B show the exterior of head 12 in schematic form. Theactual appearance of head 12, in one embodiment, is shown in FIGS. 3A to3D. FIGS. 2A and 2B show the optical paths and elements within head 12.)Head 12 comprises a laser source 14 for providing a laser beam 16. Head12 also includes a) a set of lenses, masks, mirrors and other opticalelements (described below) for modifying laser beam 16 and directing andfocusing beam 16 onto a spot on surface 13 u; and b) a set of mirrors,lenses and other optical elements (also described below) for modifyingand directing light reflected by surface 13 u to various detectors 20-26(discussed below). (As used herein, “reflected” includes specularlyreflected light and/or scattered light.) Output signals O20-O26 fromdetectors 20-26 are processed to determine the condition of surface 13u.

Head 12 is typically constructed from a monolithic block of material,and as mentioned above, it can have an exterior appearance as shown inFIGS. 3A to 3D.

In one embodiment, laser source 14 is a solid state laser (e.g. a diodelaser) having a wavelength of 660 nm. However, in other embodiments,different types of laser sources (such as a gas laser) and differenttypes of laser light (including light outside the visible range) can beused. The laser spot on surface 13 u can be circular, elliptical, or canhave another shape. In one embodiment the laser spot is substantiallyelliptical, and can be 8 μm in the azimuthal direction of platter 13,and 25 to 30 μm in the radial direction of platter 13. The laser can belinearly polarized, circularly polarized or randomly polarized.

In the exemplary apparatus of FIGS. 1A to 1C, the laser light reflectsoff of platter 13, and is directed to a deflection detector 20, a smallangle scatter detector 22, a wide-angle (approximately 90°) scatterdetector 24, and a back scatter detector 26. (Referring to FIG. 1A, thesmall angle detector 22 typically detects scattered light within a conewhose sides form an angle α with the central ray of the reflected beam,where α is a value between 5 and 30°.) Deflection detector 20 can be abi-cell detector as described in the above-mentioned '674 or '821patents. Alternatively, detector 20 can be a quad cell detector or othertype of device, e.g. for detecting laser beam deflection along one ortwo directions. Detectors 22, 24 and 26 can be photodiodes,photomultipliers, avalanche photodiodes, phototransistors, etc. In oneembodiment, an avalanche photodiode such as device model no.197-70-74-5.91, manufactured by Advanced Photonix is used. The outputsignals from detectors 20, 22, 24 and 26 are provided to associatedamplifiers A20, A22, A24 and A26, which in turn provide output signalsO20, O22, O24 and O26 to circuits C20, C22, C24 and C26, respectively,for processing. Circuits C20-C26 are described below.

During use, a spindle motor M rotates a spindle S, which in turn rotatesplatter 13. Concurrently, laser beam 16 moves in a direction of arrow A1relative to platter 13 to thereby sweep across platter surface 13 u.Accordingly, the entire usable portion of surface 13 u is scanned bylaser beam 16 to thereby inspect surface 13 u for defects. In oneembodiment, platter 13 moves in a direction A2 while it rotates, andhead 12 remains stationary. In another embodiment, head 12 moves indirection A1 while platter 13 merely rotates. In yet another embodiment,head 12 and platter 13 simultaneously move in directions A2 and A1,respectively, while platter 13 rotates. In yet another embodiment, theangle of laser beam 16 changes so that beam 16 sweeps across surface 13u while platter 13 rotates. Of importance, however, there is relativetranslational motion between laser beam 16 and platter 13 which permitssurface 13 u to be inspected. In yet another embodiment, the lasersweeps in directions along two axes while platter 13 is stationary.However, preferably, platter 13, motor M and spindle S are moved indirection A2 by a stepper motor, linear motor, or other type of motor(not shown) while head 12 remains stationary.

FIG. 1 only shows an upper test head 12 for testing top surface 13 u ofplatter 13. However, in one embodiment, a similar lower head is providedbelow platter 13 for simultaneously inspecting the bottom surface ofplatter 13 (e.g. as shown in FIG. 7). In such an embodiment, the lowerhead can be slightly different from the upper head for reasons describedbelow.

II. Detailed Description of Optical Paths of Laser Beam 16

Referring to FIGS. 2A and 2B, after laser beam 16 leaves diode laser 14,it passes through slit masks 50 and 52, reflects off a mirror 54, andpasses through optional optical elements 56. Optical elements 56comprise a glass plate 56 a, a circular polarizer 56 b (typically aquarter wave retarder) mounted on plate 56 a, and a cylindrical lens 56c having one planar surface mounted on polarizer 56 b. (Althoughelements 56 comprise structures 56 a, 56 b and 56 c affixed to oneanother, in other embodiments they need not be affixed to one another.Structures 56 a, 56 b and 56 c are separately shown schematically inFIG. 2B.)

Cylindrical lens 56 c permits control of the shape of the laser spot onplatter 13, as described below. Polarizer 56 b circularly polarizeslaser beam 16. This makes head 12 less sensitive to the direction ofscratches in platter 13, e.g. as described below.

After passing through optional elements 56, beam 16 passes through alens 58 for focusing beam 16 onto platter 13 and through a mask 60 (notshown in FIG. 2A, but shown in FIG. 2B). Beam 16 then strikes platter13. (The angle of incidence of beam 16 is typically not perpendicular toplatter 13, and in one embodiment it can be about 45° with respect toplatter 13.) As described below, some of beam 16 specularly reflects offof platter 13 and some of beam 16 is scattered by platter 13 (e.g. ifthere are defects on the platter surface). The specularly reflectedportion of laser beam 16, along with a portion of the laser lightscattered off of platter 13 at a relatively narrow angle, passes througha collecting lens 62. The light passing through collecting lens 62 ishereafter referred to as light 16-1. A portion of light 16-1 strikes asmall angled mirror 64 (not shown in FIG. 2A, but shown in FIG. 2B).Because mirror 64 is small, an outer portion 16-1′ of light 16-1 doesnot strike mirror 64, but instead travels past the outer perimeter ofmirror 64, reflects off a mirror 66, passes through a lens 68 and aniris 70 and then strikes detector 22. (Lens 68 concentrates light 16-1′on detector 22.) In this way, detector 22 receives light scattered offof platter 13 at relatively narrow angle α. (As mentioned above, portion16-1′ of light 16-1 is scattered. If platter 13 were perfectly smooth,portion 16-1′ would have an intensity of zero.)

Inner portion 16-1″ of light 16-1 reflects off of mirror 64 and strikesquad-cell detector 20 (described in greater detail below). Detector 20detects small changes in the angle of specular reflection of portion16-1″, which in turn indicate whether relatively large bumps or pits arepresent on platter 13. (Portion 16-1″ is light that specularly reflectsoff of platter 13.) Quad-cell detector 20 also detects any changes inthe amount of power of portion 16-1″. Such a change of power couldresult from fluctuation in the power provided by laser diode 14 or thepresence of an area of platter 13 that exhibits reduced reflectivity(e.g. a stain).

Although the above-described embodiment uses a small mirror 64, inanother embodiment, mirror 64 is much larger, but contains a smallopening for transmitting specularly reflected light portion 16-1″. Thetransmitted light in this embodiment (portion 16-1″) passes to quad-celldetector 20, while the mirror reflects light portion 16-1′ to detector22.

Concurrently, a portion 16-2 of the laser light scatters off platter 13,is collected by a lens 72, reflects off a mirror 74, and passes througha lens 76 which concentrates portion 16-2 onto detector 24.Concurrently, a portion 16-3 of the laser light back scatters offplatter 13, is collected by a lens 77, reflects off a mirror 78 and isconcentrated by a lens 80 onto detector 26.

As mentioned above, portions 16-1′ and 16-2 are light scattered at smalland wide angles respectively by defects in platter 13. Portion 16-3 islight that is back scattered by defects in platter 13. Portion 16-1″ islight that specularly reflects off of platter 13. Portion 16-1″indicates the angle of the walls of relatively large defects in platter13. The magnitudes of portions 16-1′, 16-1″, 16-2 and 16-3 are used todetermine various characteristics of different kinds of defects in thesurface of platter 13.

Elements 50-80 are part of head 12, and are rigidly held within anenclosure 82. In one embodiment, enclosure 82 is a monolithic block ofmaterial such as aluminum. (Although FIGS. 1 and 2A schematically showthe monolithic block as having the approximate shape of a rectangularprism, the exterior is typically as shown in FIG. 3.) Head 12 is mountedon apparatus 10 such that head 12 can be adjusted (manipulated) withthree degrees of freedom as follows: a) translational motion along the Zaxis for focusing, b) tilt about the X axis; and c) tilt about the Yaxis. Thus, head 12 can be easily manipulated without having toindividually adjust the position and angle of the optical componentswithin the head. Only a small number of adjustments is used to couplehead 12 to apparatus 10.

Although head 12 is adjustable with only three degrees of freedom, inanother embodiment other adjustments are possible. In yet anotheralternative embodiment, head 12 can be adjusted only along the Z axis.In yet another alternative embodiment, the position of detector 20and/or laser source 14 are adjustable, e.g. by using screws extendinginto head 12 (not shown) to make fine adjustments.

III. Cylindrical Lens 56 c

As mentioned above, in one embodiment, cylindrical lens 56 c is providedin the input optical path of laser beam 16. Lens 56 c facilitatescontrol of the shape of the light spot on platter 13. Typically, a laserbeam provided by a laser diode can have an aspect ratio of about 3:1. Asexplained below, laser beam 16 typically strikes platter 13 at an angle(e.g. at about 45°) so that if laser beam 16 were not otherwisemodified, the laser spot on platter 13 would have an aspect ratio ofabout 2.1:1. Lens 56 c increases the aspect ratio of beam 16. In oneembodiment, the aspect ratio of the laser spot on platter 13 is greaterthan 2.5:1, e.g. between about 4:1 and 5:1. The major axis of the laserspot is substantially parallel with direction of relative translationalmotion between platter 13 and laser beam 16, i.e. direction A1. Becauseof this, it requires less time for laser beam 16 to scan surface 13 uthan if the aspect ratio were less than 4:1. (The aspect ratio ispreferably kept at or below 5:1 because if the aspect ratio were toolarge, the energy density of the laser beam would be insufficient toadequately inspect surface 13 u.)

In one embodiment, cylindrical lens 56 c functions in two ways. First,lens 56 c reduces the beam length in the radial direction as it hitsfocusing lens 58, and thereby causes the beam length in the radialdirection to increase as it hits platter 13. Second, lens 56 c causesthe laser spot on platter 13 to be out of focus in the radial direction.(The laser spot is typically in focus in the circumferential direction.)Of importance, the combination of these two effects causes the spot sizein the radial direction to be substantially insensitive to the positionof cylindrical lens 56 c along the input optical path. For example,during experiments, one could move lens 56 c by 60 mm along the inputoptical path without altering the major axis length of the beam spot bymore than a micron.

IV. Polarization of Laser Beam 16

As mentioned above, optionally, laser beam 16 is circularly polarized,e.g. by passing beam 16 through plate 56 b. If beam 16 is linearlypolarized, the output signal from the various detectors will varydepending upon whether a scratch in surface 13 u is parallel orperpendicular to the electric field component of the laser light. Forexample, referring to FIG. 4, a scratch S1 extending along acircumferential direction of surface 13 u will have a different effecton head 12 than a scratch S2 extending along the radial direction ofsurface 13 u. For example, if a) laser beam 16 is linearly polarized; b)the plane of incidence is in the azimuthal direction, and c) thepolarization direction is in the radial direction, more light willscatter from scratch S1 than S2 (assuming the scratches are the same).If the polarization direction is in the incidence plane, more light willscatter from scratch S2 than scratch S1. (As is known in the art, theplane of incidence is the plane defined by the incident and specularlyreflected beams.) By circularly polarizing beam 16, this difference insensitivity to scratch direction is eliminated.

V. Enclosure 82 and its Method of Manufacture

As described above, head 12 comprises mirrors, lenses and other opticalelements. We have discovered a method for making head 12 in which weavoid having to individually align various optical elements within head12. We have also discovered a method for providing head 12 in a verycompact volume. For example, we have been able to construct head 12 suchthat width W (FIG. 1A) is extremely small, e.g. about 5.9″. Length L andthickness T are 6.2″ and 3.23″, respectively. (These dimensions aremerely exemplary.) This is advantageous because small variations inangles of various devices caused during manufacturing of head 12 haveless impact on the offset of optical beam paths if the size of head 12is minimized. Also, ensuring that head 12 is small permits minimizingthe “footprint” of apparatus 10.

To illustrate a method in accordance with our invention, reference ismade to FIG. 5A, which schematically illustrates a block of material 100(again, typically a metal such as aluminum). Intersecting portions 102and 103 of block 100 are milled out to form an input path for light topass from diode laser 14, to mirror 54 and to platter 13. Of importance,ledges 102 a and 102 b are left in path 102 to provide a surface forholding masks 50 and 52. Masks 50 and 52 are dropped through an opening102′ of portion 102 and glued in place.

As shown in FIG. 5B, a corner 104 of block 100 is also milled away sothat mirror 54 can be mounted at the appropriate angle on block 100,e.g. using an adhesive or other technique.

Portions 105, 106, 108 and 110 (FIGS. 3C, 3D) are also milled out ofblock 100 to form output paths for light heading toward detectors 20-26.Ledges are left in block 100 for holding lenses 68, 76 and 80.Optionally, ledges are also left in paths 106-110 to hold irises forembodiments in which such irises are used. These elements are insertedthrough openings in the milled out portions 106, 108, 110 as appropriateand glued onto their associated ledges.

Portions 111, 112 and 113 (FIGS. 3A and 3B) are also milled out of block100 to form paths for portions 16-1, 16-2 and 16-3 of the reflectedlaser light. Ledges are left in block 100 for holding collecting lenses62, 72 and 77. Lenses 62, 72 and 77 are inserted through the openings ofpaths 111, 112 and 113 and glued to their associated ledges asappropriate. In addition, a ledge 111 a is left in path 111 for holdinga glass window 64 w, upon which is affixed a pedestal 64 p, upon whichmirror 64 is mounted (FIG. 5C). (FIG. 5C merely shows one way of holdingmirror 64 within head 12. It will be appreciated that other techniquescould also be used to hold mirror 64.)

Finally, side or corner portions are cut off of block 100 so thatmirrors 66, 74 and 78 can be mounted at the appropriate angle on block100, e.g. using an adhesive or other technique.

It will be appreciated that in different embodiments, the variousportions of block 100 can be removed in an order other than as describedabove.

VI. Reduction or Elimination of Stray Light Within Head 12

In accordance with one embodiment of our invention, several techniquesare used to minimize the amount of stray light that might otherwisegenerate noise in output signals O20-O26. (Stray light can arise fromseveral sources. For example, diode lasers often emit a “halo” aroundthe main laser beam 16. Also, stray light can result from unwantedreflection off of lenses, masks or other elements within head 12.) Inone embodiment, black tubing is inserted into the various openings andapplied to the walls of the optical paths to absorb stray light therein.The surface of the tubing is blackened by an electroplating technique.(In one embodiment, nickel is electroplated onto the tubing walls. Onetype of light absorptive layer is available from Epner Technology, Inc.of Brooklyn, N.Y. See also the pages from www.epner.com submitted asExhibit A of our provisional patent application no. 60/643,748, filedJan. 13, 2005, incorporated herein by reference.) The black tubingconstitutes a “light trap” for absorbing stray light.

In an alternative embodiment, in lieu of inserting black tubing intohead 12, the interior of head 12 can be anodized to provide a dull blackmatte surface for the optical paths.

In one embodiment, a narrow band V-type AR (anti-reflective) coating isapplied to the various lenses within head 12 to prevent multiplereflections. (As used herein, the term “V-type AR coating” also includesa “Super V-Type AR coating”.) Such a coating is typically tailored tothe wavelength of laser beam 16. Reflectivity exhibited by a lens coatedin accordance with this embodiment is typically less than 0.25%.

One or more masks with slits are inserted within the optical paths oraffixed to the lenses to reduce or prevent stray light which wouldotherwise interfere with operation of head 12.

Finally, masks or irises are provided in front of one or more ofdetectors 20-26. (The irises are masks that can have an opening of anadjustable size.)

The above-mentioned masks, coatings and irises prevent or reduce straylight, e.g. light that would be present in the scattered light opticalpaths even in the absence of a defect on platter 13. These masks,coatings and irises are designed and placed to avoid impacting orsubstantially impacting light caused by defects on platter 13.

In addition, one or more other masks can be provided to block lightcaused by a desired texture or a pattern deliberately provided on thesurface of platter 13 (e.g. for discrete track recording). These masksin the output optical paths that eliminate or reduce the above-mentionedlight caused by diffraction due to patterns on platter 13 can, however,block some portion of the light caused by defects.

While antireflective coatings are provided on all lenses in oneembodiment, in another embodiment, antireflective coatings are onlyprovided on some lenses, e.g. lenses 58 and 62. Similarly, in someembodiments, light trap tubing is only placed along some of the opticalpaths, e.g. input paths 102 and 103 and small angle scatter output paths111 and 106. Also, in some embodiments, an iris is only provided infront of detector 22.

The importance of reducing stray light can be appreciated in light ofthe following. In one embodiment, an avalanche photodiode with a gain of300 is connected to a low noise transimpedance amplifier with a feedbackresistor of 10,000 ohms, followed by a post amplifier with a gain ofthree. The bandwidth of the system is 10 MHz. The measured electronicnoise was 0.45 mV RMS, while the calculated value was 0.3 mV RMS. Themeasured shot noise with 118 nW of laser light impinging on theavalanche photodiode was 7.4 mV RMS, while the calculated value was 6.2mV RMS. The shot noise is proportional to the square root of the lightpower. Therefore, in order to reduce the shot noise to the level of theelectronic noise, the stray light should be of the order of 1 nW orless. Since the typical laser power is 20 mW, one should attempt toreduce the laser stray light to 0.0005% of the laser power.

Although some embodiments include the masks, irises, tubing forabsorbing or trapping light, and/or antireflective coatings, otherembodiments lack these features.

VII. Embodiment with Reduced Number of Lenses

FIG. 6A schematically illustrates platter 13, scattered light 16-2,collecting lens 72, concentrating lens 76 and detector 22. (Mirror 74has been eliminated from FIG. 6A for ease of illustration.) If the lightbetween lenses 72 and 76 is substantially collimated, optical distanceD1 can be arbitrarily long without affecting operation of head 12. Whileone embodiment employs both lenses 72 and 76, in another embodiment, asingle lens 72′ both collects light 16-2 and concentrates light 16-2onto detector 22 (FIG. 6B). In like manner, instead of passing light16-3 through both lenses 77 and 80, a single lens can be used.Similarly, instead of passing light 16-1′ through both lenses 62 and 68,a single lens can be used.

In the embodiment of FIG. 6B, distance D1′ depends upon the focalcharacteristics of lens 72′. Distance D1′ cannot be arbitrarilyselected. Also, if lens 72′ were identical to lens 72, the distance D2between platter 13 and lens 72′ would have to be greater than thecorresponding distance D3 between lens 72 and platter 13. This wouldnecessitate a loss of effective numerical aperture for lens 72′ comparedto lens 72.

VIII. Embodiments Comprising Two Test Heads

A. Displacement of Laser Spot on Platter

As mentioned above, in one embodiment, a single test head 12 is providedfor testing upper surface 13 u of platter 13. In other embodiments, asecond test head 12 d (FIG. 7) is provided for testing the lower surfaceof platter 13 while test head 12 tests the upper surface of platter 13.In one such an embodiment, head 12 d is modified slightly to providespace for spindle S. Further, the position of lens 62 d in head 12 d isslightly offset compared to the position of lens 62 in head 12. (As usedherein, the letter “d” is added to the element reference number todistinguish between a structure in lower head 12 d and a correspondingstructure in upper head 12.) Further, whereas in one embodiment upperlaser beam 16 traces a path 122 (FIG. 8) along upper surface 13 u ofplatter 13 (i.e. in the radial direction of platter 13), lower laserbeam 16 d traces a slightly different path 122 d along lower surface 13d of platter 13 that is displaced by a distance D4 from such a radialpath. This is done to enable the use of lens 62 d with a high numericalaperture (high collection efficiency) in the presence of spindle S. (Inone embodiment, distance D4 is 4.763 mm, the diameter and back focallength of lens 62 d are 25 mm and 20.2 mm, respectively, and the minimumradius to be scanned is 15.5 mm. The effective diameter of lens 62 d is22 mm. Lens 62 is identical to lens 62 d. These values are merelyexemplary.)

FIGS. 9 and 10 illustrate a portion of head 12 d in plan view (lookingup) and side view, respectively. (In FIGS. 9 and 10, the opticalelements associated with portions 16-2 d and 16-3 d of the reflectedlaser light as well as the elements pertaining to upper surface 13 uhave been eliminated for sake of clarity.) In FIGS. 9 and 10, laser beam16 d strikes a point 124 d on surface 13 d near the inner diameter ofplatter 13. (Point 124 d is eclipsed, and therefore not visible, in FIG.10.) Lens 62 d receives a cone of light from point 124 d at an angle β.It will be appreciated that if point 124 d were not displaced fromradius R of platter 13 by distance D4, angle β would be reduced becauselens 62 d would be further away from point 124 d. Lens 62 d could not bemoved closer to point 124 d because spindle S would be in the way. Bydisplacing point 124 d in the manner shown, one need not reduce angle β,and therefore the numerical aperture of lens 62 d can be increased. Thishas the benefit of providing more light energy to detector 22 d.

The effect described above with respect to FIGS. 9 and 10 isequivalently achieved in the manner shown in FIG. 11. Referring to FIG.11, track 130 represents the innermost track of platter 13 that is to bescanned. (The term “track” is used herein includes embodiments in whichthe tracks are part of a continuous spiral and embodiments in which thetracks are discrete.) Laser beam 16 d strikes point 124 d on platter 13such that the plane of incidence is not tangential with track 130.Rather, in the illustrated embodiment, the plane of incidence forms anangle θ (in one embodiment, 17.90°) with respect to track 130. Byarranging the plane of incidence and track 130 as described above, onecan arrange lens 62 d so that track 130 can be scanned withoutsacrificing numerical aperture.

It is noted that the cone of light of incident laser beam 16 d is muchnarrower than the cone of reflected light 16-1 d. This characteristic ofthe incident and reflected beams enables being able to employ a high NAfor lens 62 d using the displacement technique discussed above.

As mentioned above, in one embodiment laser beams 16, 16 d scan top andbottom surfaces 13 u, 13 d of platter 13 simultaneously. It is moreimportant that lower laser beam 16 d is displaced in the mannerdiscussed above (or having the plane of incidence intersect with track130 as discussed above) because spindle S (which extends below, butgenerally not above, platter 13) interferes with placement of lens 62 d.Thus, displacement of the upper laser beam 16 is unnecessary. Althoughunnecessary, in some embodiments upper laser beam 16 is displaced sothat heads 12 and 12 d can be substantially identical. In fact, FIGS. 3Ato 3D show a half-cylindrical section 132 cut out of head 12. This isuseful if heads 12 and 12 d are to be identical, and it allows room sothat head 12 d can accommodate spindle S when scanning the inner tracksof surface 13 d.

Because beam 16 d is displaced, the software processing the outputsignals from detectors 20 d to 26 d takes into account this displacementwhen generating a “map” of the characteristics of surface 13 d asdescribed below.

B. Angle of Laser Beam 16 With Respect to Beam 16 d

In one embodiment, heads 12 and 12 d are arranged to avoid or minimizeinterference of laser beam 16 on head 12 d, and to avoid or minimizeinterference of laser beam 16 d on head 12. This can be accomplished byselection of the angle of incidence of laser beams 16 and 16 d onplatter 13. FIG. 12 shows in plan view the juxtaposition of heads 12, 12d and platter 13. FIG. 12 also indicates a) direction A2 of travel ofplatter 13, b) the direction A3 of incident laser beam 16 (after leavingmirror 54) and specularly reflected laser beam 16″ (prior to strikingmirror 64) in the plane of platter 13, and c) the direction A4 ofincident laser beam 16 d (after leaving mirror 54 d) and specularlyreflected laser beam 16 d″ (prior to striking mirror 64 d) in the planeof platter 13.

As can be seen directions A3 and A4 are at an angle γ such that beams16″ and 16 d″ travel in different directions. (Angle γ is typicallygreater than 0° but less than 20°.) This angle reduces the probabilitythat light scattered from one side of platter 13 will travel to thecollection lenses on the other side of platter 13 when beam 16 is nearthe outer edge of the platter. Advantageously, this prevents “crosscommunication” or interference between laser light in head 12 fromaffecting head 12 d and vice versa.

As can be seen in FIG. 12, directions A3 and A4 are somewhat close tobeing antiparallel. Thus, in the embodiment of FIG. 12, heads 12 and 12d can be identical, with head 12 d being a “flipped over” version ofhead 12. The facts that a) directions A3 and A4 are close toantiparallel; b) beam 16 travels generally away from mirror 64 d; and c)beam 16 d travels generally away from mirror 64 also serve to preventcross-interference between heads 12 and 12 d (especially when beams 16and 16 d strike points close to the outer diameter of platter 13).However, in another embodiment, however, directions A3 and A4 need notbe close to antiparallel.

FIG. 12 shows that points 124, 124 d are displaced from one another bydistance D4 (discussed above). However, in an alternate embodiment, spot124 is directly over spot 124 d. In yet another embodiment, spots 124and 124 d are displaced by a distance of two times D4. (In such anembodiment, heads 12 and 12 d can be identical.)

Although directions A3 and A4 form angle γ, in one embodiment,directions A3 and A4 are antiparallel. In yet another embodiment,directions A3 and A4 are closer to parallel than antiparallel, but stillform an angle γ with respect to one another.

IX. Quad Detector 20 and Circuit C20

In one embodiment, detector 20 is a quad detector 20 such as asemiconductor device having four regions 20-1, 20-2, 20-3 and 20-4 (FIG.13). A spot 150 of portion 16-1″ of laser light 16 strikes detector 20as described above. If laser beam 16 strikes a defect on surface 13 u,depending upon the slope of the defect walls, spot 150 will be deflectedaway from the center C of detector 20. If spot 150 is deflected in adirection A5, the output signal from region 20-2 will be greater thanthe output signals from regions 20-1, 20-3 and 20-4. If a steeper defectis encountered by beam 16 on surface 13 u, spot 150 will move further indirection A5, and the output signal from region 20-2 will be greaterthan it would have been if a less steep defect was encountered.

If a defect causes spot 150 to be deflected in direction A6, the outputsignal from region 20-1 will exceed that of regions 20-2 to 20-4. Inthis way, detector 20 provides signals to circuit C20 indicating thedirection and steepness of a wall of a defect on surface 13 u.

In one embodiment, circuit C20 includes analog circuits 200, 201 and 202(FIG. 14) which generate signals S200, S201 and S202 proportional to(I1+I2), (I3+I4) and (I1+I2)−(I3+I4), respectively, where I1, I2, I3 andI4 are the amounts of light striking regions 20-1, 20-2, 20-3 and 20-4,respectively. Signal S202 indicates the extent to which portion 16-1″ oflaser light is deflected upward or downward, e.g. by defects in thesurface of platter 13. Circuit C20 also includes analog circuits 203,204 and 205, which generate signals S203, S204 and S205, proportional to(I1+I3), (I2+I4) and (I1+I3)−(I2+I4), respectively. Signal S205indicates the extent to which portion 16-1″ of the laser light isdeflected to the left or right. Circuit C20 also includes analog circuit207 which generates an electrical signal S207 having a voltageproportional to (I1+I2+I3+I4). Signal S207 represents the total amountof light striking detector 20. Signals S202 and S205 are typicallynormalized (e.g. divided by) the value I1+I2+I3+I4 (S207) so that theapparatus is insensitive to laser output power fluctuations andreflectivity variations of regions of platter 13. Normalization can bedone using analog techniques. However, signals S202, S205 and S207 canalso be digitized (e.g. using an analog to digital converter), andnormalization can be done digitally, e.g. using a computer as describedbelow.

As mentioned above, in one embodiment, circuits 200-207 sum and subtractvarious signals using analog techniques. However, in other embodiments,signals I1 to I4 can be digitized, and the summing and subtraction canalso be done digitally. As discussed above, the summing and subtractionare typically done in a plurality of stages (e.g. summing first,subtracting second). However, in other embodiments, these functions canbe performed in one stage.

X. Description of Circuits C22, C24 and C26

Circuits C22, C24 and C26 (for processing the output signals ofdetectors 22-24) are identical. FIG. 15A illustrates circuit C22. As canbe seen, output signal O22 from detector 22 is provided to ananalog-to-digital converter 300, which provides its output data to aFIFO memory 302. The output of FIFO memory 302 can be asynchronouslyread by a microprocessor 304. In this way, microprocessor 304 reads datafrom FIFO memory 302 corresponding to the output signal provided bydetector 22. Microprocessor 304 provides these data to a general purposeprocessor 306 via a high speed bus interface circuit 308 and a bus 310(in one embodiment a USB bus). (Processor 306 then performs additionalprocessing on the data from FIFO memory 302.) In one embodiment,microprocessor 304 is a device such as model C8051F120, available fromSilicon Laboratories, and has a clock speed of 100 MHz. However, othermicroprocessors can also be used. Also, while processor 306 is typicallya general purpose processor, alternatively, it can be a digital signalprocessor, e.g. a Texas Instruments TMS320F2812, which can operate at150 MIPS.

Interface circuit 308 receives data on a set of buses 312, which aredriven by microprocessors in circuits C20, C24 and C26 similar tomicroprocessor 304.

Advantageously, in one embodiment, digital values of signal O22 are onlystored in FIFO memory 302 when signal O22 exceeds a threshold signalTHR. This is an advantage because it enables efficient use of FIFOmemory 302 by storing only data that are of interest for evaluatingcharacteristics of platter 13. Thus, by only selecting these digitalvalues, the memory and processor requirements of circuit C22 andprocessor 306 are reduced. The manner in which this is accomplished isas follows. As seen in FIG. 15A, a write data enable signal DEN for FIFOmemory 302 is provided by an analog comparator 316. Signal DEN is onlyactive when signal O22 from detector 22 exceeds threshold signal THR.Thus, data are only stored in FIFO memory 302 when signal O22 exceedssignal THR.

Signal THR is an analog signal generated by digital to analog converter318, which in turn is controlled by microprocessor 304. Thus,microprocessor 304 controls the magnitude of signal THR. Signal THR isuser-selectable so that only events of interest are passed. (In general,signal THR is made to be greater than the signal noise level.)Optionally, in one embodiment, microprocessor 304 or processor 306establishes the value of signal THR in response to the measured noisepresent in signal O22.

It is typically desirable to provide location data to processor 306indicating the location on platter 13 that causes signals O22-O26 toexceed their associated threshold values THR. In one embodiment, this isdone by providing a “track address” (identifying the position on surface13 u in a radial direction) and a “sector address” (identifying theposition on surface 13 u in a circumferential direction) to processor306 where the conditions of surface 13 u cause signal O22 to exceedthreshold signal THR. In one embodiment, circuitry is provided whichindicates the start location (track and sector address) of an area onsurface 13 u where signal O22 begins to exceed signal THR, and the endlocation on surface 13 u where signal O22 falls below signal THR.

FIG. 15B illustrates address circuit 400 for providing the start andstop addresses. Circuit 400 includes a spindle index input lead 402 anda sample clock input lead 404. Spindle index input lead receives a pulseevery time platter 13 completes a revolution. These pulses are countedby a counter 406, which provides a track address on a track address bus408 in response thereto. The track address is the number of the trackcurrently scanned by laser beam 16.

A second counter 410 receives input pulses from sample clock input lead404. In one embodiment, these pulses are provided at a rate of 249,856pulses per platter revolution, although this number is merely exemplary.The sample clock pulses are synchronized with the disk rotation. In oneembodiment, this is accomplished by providing an optical spindle encoderschematically represented as box 412 coupled to spindle S. This encoderprovides 512 pulses per platter revolution. A clock circuit 414 iscoupled to receive these pulses and generate the sample clock pulses inresponse thereto using a phase-locked loop to create an in-phasemultiple of the spindle encoder pulses. Counter 410 counts the sampleclock pulses and provides a sector address on a bus 418.

The track and sector addresses are stored in FIFO memories 420 and 422,respectively in response to signal AEN. (Signal AEN goes active whensignal O22 first exceeds threshold THR, and again when signal O22 fallsbelow threshold THR. Thus, signal AEN represents the beginning and endlocations of a defect on surface 13 u.) Microprocessor 304asynchronously reads the track and sector addresses from FIFO memories420 and 422 via a bus 424. These addresses are then provided bymicroprocessor 304 to processor 306 via interface circuit 308 and bus310.

As mentioned above, separate counters 406, 410 are used to generatetrack and sector addresses. However, in an alternate embodiment, asingle counter can be used to generate both the track and sectoraddresses. (In one such an embodiment, the sector address is Q modulusN, where Q is the value stored in the single counter and N is the numberof sectors per revolution. The track address is the integer portion ofQ/N.) The output from this single counter can be provided to a FIFOmemory of appropriate width. (Alternatively, one can construct a counterin which the lower counter bits constitute the sector address, the uppercounter bits constitute the track address and only increment when thelower bits reach the value N, and the lower bits reset upon reaching N.)

As mentioned above, signal O22 in analog form is compared to thresholdsignal THR. However, in an alternative embodiment, signal O22 isdigitized (or otherwise provided in digital form) and threshold THR isin the form of a digital value. The digitized signal O22 is comparedwith this digital threshold THR value to generate signals AEN and DEN.

Although in some embodiments, signal THR can be changed bymicroprocessor 304, in other embodiments, signal THR cannot be changed.

While the above-described embodiments pass information to the variousFIFO memories whenever signal O22 exceeds signal THR, in someembodiments, information is only passed to the FIFO memories when signalO22 is less than an upper threshold value. In yet another embodiment,information is only passed to the FIFO memories when signal O22 issimultaneously greater than signal THR and less than an upper thresholdvalue. In yet another embodiment, information is only passed to the FIFOmemories when signal O22 is either less than signal THR (a lowerthreshold) or greater than an upper threshold value.

As mentioned above, circuit C20 (which processes output signals fromquad detector 20) provides output signals S202 and S205 (correspondingto the vertical and horizontal deflection of portion 16-1″ of laser beam16) and signal S207 (corresponding to the total amount of power withinportion 16-1″). Signals S202 and S205 are provided via an amplifier tocircuits that are substantially identical to two iterations of circuitC22 (one for signal S202, and one for signal S205), where they aredigitized and passed to microprocessor 304 and processor 306 if theyexceed associated threshold voltages.

Signal S207 is passed to microprocessor 304 of circuit C20 and processor306 whenever signal S202 or S205 is converted to a digital value, storedin a FIFO memory which is asynchronously read by microprocessor 304 andpassed on to processor 306. In this way, signals S202 and S205 can benormalized by processor 306.

XI. Detailed Description of an Embodiment of Circuits C22-C24

FIGS. 16A and 16B illustrate in greater detail the structure of FIGS.15A and 15B. In FIGS. 16A and 16B, WRclk, Wren, WR Prt, RDclk, Rden andRD Prt refer to the write clock, write enable, write port, read clock,read enable and read port. The counter input CIR resets counters 406 and410. The input Din increments counters 406 and 410. Signal SCLK isgenerated from clock circuit 414. FIG. 16C shows a write enable sequencelogic circuit 426 that can be used to generate signals DEN and AEN.Circuit 426 is useful because analog to digital converter 300 typicallyexhibits a finite pipeline delay of several cycles of clock signal SCLK.Circuit 426 facilitates proper loading of the FIFO memories given thispipeline delay. As shown in FIG. 16C, in one embodiment circuit 426comprises a network of serially connected D-type flip-flops 427 thatgenerate signals DEN and AEN in such a manner that these signals only goactive if signal O22 exceeds threshold signal THR for a certain amountof time. (As mentioned above, signal DEN enables writing data to FIFOmemory 302. Signal AEN enables writing data to FIFO memories 406 and410.) Each of D Flip-flops 427 stores the out put signal of comparator316 delayed by an associated number of pulses of clock signal SCLK.Circuit 426 provides two gating signals, one for generating signal DENand one for generating signal AEN. The timing for signals DEN and AENare generated from the output signal of comparator 316 and the pulsetrain that constitutes signal SCLK. In particular, signals DEN and AENare based on selected states of the flip-flops 427. (For example, signalDEN is the output of comparator 316 delayed by four pulses of clocksignal SCLK. Signal AEN comprises one pulse on the rising edge of signalDEN and one pulse immediately before signal DEN falls.) The circuitshown in FIG. 16C is merely exemplary.

XII. Robotics Used in Conjunction with Head 12

FIG. 17A schematically illustrates a “cell” 500 for testing platters ina manufacturing environment. A conveyor 502 provides cassettes 504 ofplatters 13 to be inspected. A robotic arm 506 provides platters 13 toeither test apparatus 10 a or test apparatus 10 b. (Both apparatus 10 aand 10 b typically comprise both upper and lower heads 12, 12 d fortesting both sides of platters 13.) After test apparatus 10 a tests aplatter 13, arm 506 grabs platter 13 and places it either in a rejectcontainer 508, a pass cassette 510, or a further evaluation cassette 512a. (Typically, the vast majority of platters, e.g. more than 90% of theplatters, should be placed in pass cassette 510.) The platters depositedin reject container 508 are not necessarily handled with the degree ofcare that would be needed if the platters were to be subjected tofurther manufacturing steps. However, the platters in cassettes 510 or512 a are typically maintained with such care. After arm 506 moves aplatter 13 from apparatus 10 a to one of cassettes 510 or 512 a orcontainer 508, arm 506 takes another platter 13 from cassette 504 andplaces it in test apparatus 10 a to be evaluated.

In like manner, after test apparatus 10 b evaluates a platter 13, arm506 places that platter either in reject container 508, or one ofcassettes 510 or 512 a, depending upon the outcome of testing.Thereafter, arm 506 takes another platter from cassette 504 and placesit in apparatus 10 b.

Advantageously, container 508 has a very large capacity and does notneed to be replaced often. Thus, it is unnecessary to shut down cell 500very often to empty container 508.

Although FIG. 17A shows robotic arm 506 servicing two test apparatuses10 a, 10 b, in alternative embodiments, cell 500 may comprise one ormore test apparatuses, with arm 506 servicing the additionalapparatuses. Also, although FIG. 17A shows cassette 512 a for furtherevaluation, in other embodiments, several cassettes can be provided forreceiving platters that are to be subjected to further evaluation (e.g.optional cassettes 512 b and 512 c, shown in phantom). Platters 13 maybe placed in different ones of these cassettes depending upon specificcharacteristics of their surfaces as determined by test apparatus 10 a,10 b. Also, although FIG. 17A shows one reject container 508, in otherembodiments, a plurality of reject containers 508 can be provided. Also,although FIG. 17A shows only one arm 506, a plurality of arms can beprovided in cell 500 for moving platters. For example, one of the armscan provide platters to test apparatus 10 a, 10 b, and another arm canprovide platters to container 508 and/or cassettes 510 and 512 a.

A safety enclosure 513 surrounds cell 500 to prevent injury tomanufacturing personnel. In one embodiment, conveyor mechanisms 502 and511 continuously carry new cassettes 504 and 510 into the area protectedby enclosure 513.

An alarm 514 a indicates if cassette 512 a is full. Alarm 514 a canprovide an audible signal. Alternatively, alarm 514 a can be a lightthat illuminates to indicate that cassette 512 a is full. (Alarm 514 acan be actuated either by a sensor that determines that cassette 512 ais full, or by a counter that determines that cassette 512 a is full bycounting the platters therein. Alarm 514 a can be an LED, a smallincandescent bulb, or other optical display element.) However, duringthe time between the actuation of alarm 514 a and replacement ofcassette 512 a, robotic arm 506 places platters that would otherwise beplaced in cassette 512 a into container 508. In this way, while cassette512 a is full, cell 500 need not be turned off.

Of importance, cassette 512 a is within a drawer 515 a. When cassette512 a is full, it can be replaced by operating personnel by openingdrawer 515 a to thereby take cassette 512 a outside of the areaprotected by enclosure 513. Thus, it is unnecessary to open enclosure513 when replacing cassette 512 a. This also reduces the amount of timerequired to replace cassette 512 a, and in particular, facilitatesmaking it unnecessary to shut down cell 500 when replacing cassette 512a. (Optional cassettes 512 b and 512 c are similarly situated in drawers515 b, 515 c, and are removed from enclosure 513 in like manner. Alarms514 b, 514 c (similar to alarm 514 a) inform the machine operator whencassettes 512 b, 512 c are full, but during the interval between thetime cassettes 512 b, 512 c are full and the time they are replaced,platters that would otherwise be deposited in cassettes 512 b, 512 c areplaced in container 508.)

Typically, at least some of the platters normally placed in cassette 512a are recyclable. For example, they can be re-polished and then used.Alternatively, they can be sent for further failure analysis. Althoughthese platters may be useful, placing them into reject container 508 isnot so critical that it is worth shutting down cell 500 while waitingfor cassette 512 a to be replaced, especially since a fairly smallpercentage of the platters would be placed in cassette 512 a.

In one embodiment, cell 500 is controlled by a control circuit such as amicroprocessor or a microcontroller (not shown).

Although cell 500 includes container 508 within enclosure 513, inanother embodiment, container 508 is outside enclosure 513, and a chute(not shown) extends from inside enclosure 513 to container 508. Roboticarm 506 drops platters into the chute, and they drop into container 508.This facilitates easy and quick replacement of container 508 when itbecomes full.

FIG. 17B illustrates an alternative embodiment of a cell 500′ in which acassette 516 is provided on a conveyor 518. Cassette 516 providesplatters to be tested by cell 500′. If the platters pass testing, theyare placed back in cassette 516. In this way, one cassette is used asboth an input and output cassette. Conveyor 518 provides a steady flowof cassettes into and out of the area protected by enclosure 513.

While the invention has been described in detail, those skilled in theart will appreciate that changes can be made in form and detail withoutdeparting from the spirit and scope of the invention. For example, ahead in accordance with our invention can include more or fewer opticalelements than described above. Different types of optical elements canbe included in the path of the incident or reflected light. Differentnumbers of incident light paths can be used. Also, different numbers ofreflected light paths (e.g. one to six) can be used.

Although the above-described heads are made from a monolithic block ofmaterial, in other embodiments, the heads are not made from a monolithicblock of material. Also, although the above-described apparatuscomprises two laser sources 14, 14 d, one for top head 12 and one forthe bottom head 12 d, in another embodiment, a single laser source canbe used in conjunction with a beam splitter to provide two laser beams.Also, apparatus in accordance with the invention can be used to inspectdifferent types of workpieces. One or more of the different featuresdescribed above can be used without the other features described above.

In one embodiment, collecting lenses 62, 72 and 77 can collimate lightpassing therethrough. However, in other embodiments, the light passingthrough these lenses need not be completely collimated. Similarly,lenses 68, 76 and 80 concentrate light. Optionally, these lenses mayfocus light on detectors 22-24, but this is not absolutely necessary.Accordingly, all such changes come within the invention.

1. Apparatus comprising: a first optical head for inspecting a firstsurface of a planar workpiece, said first optical head comprising afirst light source for providing a first light beam in a first directiontoward said first surface and a first detector for detecting reflectedlight from said first surface, said first direction not beingperpendicular to said first surface; a second optical head forinspecting a second surface of said workpiece, said second optical headcomprising a second light source for providing a second light beam in asecond direction toward said second surface and a second detector fordetecting reflected light from said second surface, said seconddirection not being perpendicular to said second surface; the planes ofincidence of said first and second light beams having the characteristicthat if said workpiece is not present, said first light beam does nottravel toward said second detector and said second light beam does nottravel toward said first detector.
 2. Apparatus of claim 1 wherein theplanes of incidence of said first and second light beams are closer toantiparallel than parallel.
 3. Apparatus of claim 1 wherein saidworkpiece is a platter and said first and second surfaces are inspectedsimultaneously.
 4. Apparatus comprising: a first optical head forinspecting a first surface of a planar workpiece, said first opticalhead comprising a first light source for providing a first light beam ina first direction toward said first surface and a first detector fordetecting reflected light from said first surface, said first directionnot being perpendicular to said first surface; a second optical head forinspecting a second surface of said workpiece, said second optical headcomprising a second light source for providing a second light beam in asecond direction toward said second surface and a second detector fordetecting reflected light from said second surface, said seconddirection not being perpendicular to said second surface; the planes ofincidence of said first and second light beams being at an angle toreduce or eliminate cross-optical-coupling between said first and secondheads when said workpiece is not present or when one or both of saidfirst and second light beams approaches an outer edge of said workpiece.5. Apparatus of claim 4 wherein said angle prevents or reduces theamount of light from said first beam from reflecting off of an objectand reaching said second detector when said first light beam approachesan outer edge of said workpiece, said angle also preventing or reducingthe amount of light from said second beam reflecting off of an objectand reaching said first detector when said second light beam approachesan outer edge of said workpiece.
 6. Apparatus of claim 5 wherein saidfirst detector receives light scattered by said first surface and saidsecond detector receives light scattered by said second surface. 7.Apparatus of claim 5 further wherein said first head comprises a firstlens for collecting light reflected from said first surface of saidworkpiece and a first mirror for reflecting said collected light towardsaid first detector, and wherein said second head comprises a secondlens for collecting light reflected from said second surface of saidworkpiece and a second mirror for reflecting said collected light towardsaid second detector.
 8. Apparatus of claim 5 wherein said first headfurther comprises a first plurality of lenses for collecting lightreflected from said first surface and a first plurality of detectors forreceiving said collected light, said second head further comprising asecond plurality of lenses for collecting light reflected from saidsecond surface and a second plurality of detectors for receiving saidcollected light.
 9. Apparatus of claim 4 wherein said workpiece is aplatter.
 10. A method comprising: providing a first beam of light in afirst direction toward a first side of a workpiece such that said firstbeam reflects off of said first side and strikes a first detector;providing a second beam of light in a second direction toward a secondside of said workpiece such that said second beam reflects off of saidsecond side and strikes a second detector, the planes of incidence ofsaid first and second light beams having the characteristic that if saidworkpiece is not present, said first light beam does not travel towardsaid second detector and said second light beam does not travel towardsaid first detector.
 11. Method of claim 10 wherein the planes ofincidence of said first and second beams of light are closer toantiparallel than parallel.
 12. Method of claim 10 wherein saidworkpiece is a platter.
 13. A method comprising: providing a first beamof light in a first direction toward a first side of a workpiece suchthat said first beam reflects off of said first side and strikes a firstdetector; providing a second beam of light in a second direction towarda second side of said workpiece such that said second beam reflects offof said second side and strikes a second detector, said first and secondheads arranged so that the planes of incidence of said first and secondlight beams are at an angle to reduce or eliminatecross-optical-coupling between said first and second heads when saidworkpiece is not present or when one or both of said light beamsapproaches an outer edge of said workpiece.
 14. Method of claim 13wherein said angle prevents or reduces the amount of light from saidfirst beam reflecting off an object toward said second detector and theamount of light from said second beam reflecting off an object towardsaid first light detector.
 15. Method of claim 13 wherein said firstdetector receives light scattered by said first surface and said seconddetector receives light scattered by said second surface.
 16. Method ofclaim 13 wherein said workpiece is a platter.